The subcutaneous route is one of the most versatile routes of administration in that it can be used for both short term and very long term therapies. Additional benefits of subcutaneous administration include favorable bioavailability and pharmacokinetic properties and ease of delivery independent of the consciousness or rationality of the patient. While the subcutaneous tissue is an important target of drug delivery, it can be difficult to determine the boundaries of the subcutis between the dermal layers and underlying muscle tissue. For many medications, accidental administration within the cutis or the muscle tissue can lead to side effects, incorrect dosage or leakage of the medication from the injection site.
For example, insulin accidentally injected into the muscle tissue will be absorbed more rapidly than if injected into the subcutaneous tissue and this is potentially dangerous as it may cause low levels of blood glucose. Other therapies, such as immunoglobulins, can irritate mast cells if injected into the dermal layers. For these therapies, clearing the dermal layers is important to avoid reactions which can include swelling, redness, itching, pain, and in some cases, the drug leaking out of the injection sites due to the swelling surrounding the area.
The injection into the subcutis can be controlled by the length of the needle used for the injection or infusion. However, it is often difficult to know the required length of needle needed to clear the cutis given the variability in body fat compositions at different locations in the body. Skin irregularities such as scar tissue add even more complexity to the determination of cutis depth.
Some groups have looked at methods of localizing insulin injections in the subcutis by determining muscle depth. Using ultrasonic means, the maximum allowable penetration length of a needle may be determined to avoid injection of insulin into the muscle and the accompanying negative side effects.
There remains a need for a reliable, accurate and inexpensive way to determine depth of the cutis to avoid injection of a potential irritant, such as an immunoglobulin, into the dermal tissue.